The invention relates to a power amplifier with at least one power transistor.
The invention furthermore relates to a method for operating a power amplifier which contains at least one power transistor.
Transistor amplifiers are used even at very high frequencies When designing the circuit, it is known for consideration to be given not only to the transistor characteristics at high frequencies but also to the radio-frequency characteristic of components and lines. All the complex four-pole parameters of the transistor are thus taken into account in radio-frequency circuits.
It is furthermore known, when designing integrated radio-frequency power amplifiers, for it to be necessary to find a compromise between a high voltage resistance (beakdown) and a high switching speed (high efficiency). If it is intended to achieve good characteristics at low operating voltages, a high transit frequency is required, with a high current gain.
The article xe2x80x9cImpact-lonization Induced Instabilities in High-Speed Bipolar Transistors and their Influence on the Maximum Usable Output Voltagexe2x80x9d by M. Rickelt and H.-M. Rein describes that the breakdown characteristic of a bipolar transistor depends on the doping and thickness of a collector region of the transistor. The article furthermore describes that a collector current Ic rises sharply as the collector-emitter voltage VCE rises (breakdown). Circuits for setting quiescent currents in a transistor are known from Meinke, Gundlach: Taschenbuch der Hochfrequenztechnik [Radio Frequency Technology Manual], 5th edition, Berlin, Springer, pages F34-F35, 1992.
Furthermore, a radio frequency amplifier circuit using power transistors is known from DE 35 86 368 T2. In this known amplifier circuit, a path which conducts direct current is provided between the operating potential source and the reference ground potential, through a first transistor and a second transistor connected in series. The operating potential of the source is in consequence split between the transistors on the basis of the detect-current series path, so that each transistor has an equal share of the direct current power, while separate, parallel alternating current signal paths are provided by each transistor, and are combined at the output.
The invention is bad on the object of providing a power amplifier of this generic type, which has high gain. The power amplifier according to the invention should preferably have a transit frequency which is as high as possible.
According to the invention, this object is achieved in that a power amplifier of this generic type is designed such that the power transistor is connected such that it is operated in the breakdown region, and such that a control loop is provided, by means of which charge carriers produced in the breakdown are carried away from one output of the operational amplifier.
The invention provides for a power amplifier with one or more power transistors to be operated such that at least one of the power transistors is in a breakdown region. The breakdown region is designed such that the respective power transistor is not destroyed by it.
The invention makes use of the fact that the gradient of the collector current Ic is dependent on the auxiliary voltage (bias voltage). A change in the source impedance of the bias drive, in particular a low-impedance bias drive, considerably reduces the rise in the collector current Ic, so that the operational reliability is improved considerably, even at high operating voltages.
It is particularly advantageous to design the power transistor such that the control loop contains at least one transistor.
One particularly expedient embodiment of the power amplifier, or of the method for its operation, is distinguished in that the transistor in the control loop is connected in parallel with the power transistor.
It is particularly advantageous to design the power amplifier, or to carry out the method for its operation, such that the control loop has a long time constant in comparison to a time constant for charge carrier multiplication, and combination with at least one diode results in improved operational reliability in the breakdown region.
The subject matter of the invention is furthermore to carry out a method for operation of at least one transistor such that the transistor is operated at a constant operating point Ic.
In this case, it is advantageous for the operating point to be set by means of a control loop. The loop gain of the control loop is preferably kept low, in order to provide better stability. If driven by radio frequency signals, a rise in a mean quiescent current is expediently deliberately allowed. This makes it possible for the power amplifier to operate more efficiently at radio frequencies. For example, in the case of a radio frequency drive, a rise in the mean base voltage by, for example, 25 mV results in the collector current rising by a factor of 2.7.